2 and 2) in Linear Oligo- and

Article pubs.acs.org/JPCB

Formation of High-Spin States (S = 3/2 and 2) in Linear Oligo- and Polyarylamines Łukasz Skórka,† Jean-Marie Mouesca,‡,§ Lionel Dubois,‡,§ Ewa Szewczyk,† Ireneusz Wielgus,† Vincent Maurel,*,‡,§ and Irena Kulszewicz-Bajer*,† †

Faculty of Chemistry, Warsaw University of Technology, Noakowskiego 3, 00-664 Warsaw, Poland Universite Grenoble Alpes, INAC, SCIB, F-38000 Grenoble, France § CEA, INAC, SCIB, F-38054 Grenoble, France ‡

S Supporting Information *

ABSTRACT: This article describes the study of a linear trimer and three polyarylamines PB1−3 containing a 3,4′-biphenyl ferromagnetic coupler. The synthesis of the model compound (trimer) and the polymers has been presented. The formation of radical cations was studied using electrochemical and optical (UV−vis) methods. The chemical oxidation of these compounds leads to the creation of high-spin states, evidenced by pulsed EPR nutation spectroscopy. A quartet spin state is observed for the trimer model compound, and its J exchange coupling constant has been measured experimentally (J/k = 11.8 K) and compared quantitatively to DFT calculations. Most importantly, quartet and quintet spin states have been formed for PB3 and PB2, respectively. These last two doped polymers thus exhibit the highest spin states observed to date for linear polyarylamine compounds.

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INTRODUCTION Successful applications of conjugated molecules in OLED,1,2 field effect transistors,3−5 and photovoltaic cells6−8 motivate the search of purely organic materials exhibiting magnetic ordering. This type of materials could be more easily processable than classical inorganic magnets and could be used in electronic devices. The design of magnetic organic compounds relies on mastering the interplay between two alternating building blocks, namely, “spin-bearing units” and “spin-coupling units”. 9−11 Spin-bearing units are bearers of unpaired electron(s) of nonzero resulting spin, for example, in the form of (neutral or ionic) radicals. The simplest spin-bearing unit would have a spin 1/2. Several chemical moieties have been used as spin 1/2 sources, for example, (neutral) triphenylmethyl radicals,12 aminyl radicals,13−15 phenoxyl radicals,16 nitroxides,17,18 nitronyl nitroxides,19,20 or (cationic) aryl amminium radicals.21−25 Carbenes have also been used as source of spin 1.26−28 Each of these spin-bearing units can be characterized by the spatial extension of its (unpaired) spin orbital, hence called “magnetic” orbital, over the moiety. spincoupling units chemically couple spin-bearing units while ensuring that magnetic exchange interaction occurs between magnetic orbitals. This results, in favorable cases, in ferromagnetic coupling and, therefore, higher spin states for spin-coupled systems. The strength of the magnetic interaction © 2015 American Chemical Society

is measured by the so-called exchange coupling constant J, being the sum of two opposite contributions, one ferromagnetic (JF) and one antiferromagnetic (JAF): J = JF + JAF. Therefore, two goals have to be pursued at once. First, JAF being proportional to the squared overlap between the magnetic orbitals, the spin-coupling unit must be designed to ensure that this overlap is as small as possible (orthogonality condition). Second, the magnitude of JF (and therefore that of J if JAF ≈ 0) is directly linked to the spatial coextensivity of the magnetic orbitals (i.e., occupying the same regionthat of the spincoupling unitwithout overlapping). The seminal report by Ovchinnikov29 about alternating conjugated hydrocarbons and later DFT calculations performed by Li et al. showed that highly effective ferromagnetic spin-coupling units are 1,3phenylene or 2,7-naphthalene moieties and their derivatives.30,31 Finally, it can be noticed that magnetic coupling interactions (and therefore |J|) are expected to decrease with increasing sizes of the couplers and/or bearers. Thus, the strongest and most frequently used ferromagnetic coupler is the 1,3-phenylene unit due to large coextensivity and orthogonality of the magnetic orbitals. Received: August 28, 2015 Revised: September 28, 2015 Published: September 28, 2015 13462

DOI: 10.1021/acs.jpcb.5b08390 J. Phys. Chem. B 2015, 119, 13462−13471

Article

The Journal of Physical Chemistry B Scheme 1. Chemical Structures of the Studied Oligoarylamines and Polyarylamines Built with 3,4′-Biphenyl Couplers

of linear polyarylamines, we should also consider Coulombic effects on local conformations of these polymer chains around the spin couplers as this also can change the spin interaction mode from a ferromagnetic to antiferromagnetic one. In linear polyarylamines, ferromagnetic coupling between adjacent spins has been observed but including only two units (i.e., with resulting spin S = 1).34,35 Extending this spin interaction beyond S = 1 along polymer chains has not been detected to date and constitutes a challenge that we try to meet in the present work. Bushby and co-workers have shown that the simplest polymer from the polyarylamine family, namely, poly(maniline), cannot be effectively oxidized due to (see above) an important increase of the oxidation potential induced by electrostatic repulsion.33 Bushby postulated therefore that separating arylamine sites by 3,4′-biphenyl moieties would decrease Coulombic repulsion enough to obtain high-spin (HS) compounds.32 Recently, we have synthesized and studied arylamine model compounds (dimer d and trimer t) relying on 4-butylphenylamine spin bearers connected by the 3,4′biphenyl spin coupler.36 As it turned out, it became possible to oxidize effectively all amine sites into radical cations and to observe magnetic spin couplings yielding triplet and quartet

Taking into account potential applications of organic magnetic materials, we should consider the chemical stability of the units used as building blocks. For this reason, arylamines seem to be very promising because they can easily form radical cations relatively stable at room temperature, which can be used as spin-bearing units. However, in the case of these compounds comprising amminium radical cations, we should consider not only the building-up of coextensive magnetic orbitals,32,33 as mentioned above, but also the electrostatic repulsion between adjacent positive charges. This charge repulsion can cause an important increase of the oxidation potentials of amine sites next to already created radical cations. In turn, this can lead to the formation of spin defects (i.e., nonoxidizable, and therefore nonmagnetic, sites). Relieving this electrostatic repulsion can be achieved in two ways, either by increasing the conjugation length of the spin-bearing arylamine unit, with the risk of leaving little spin density spilling over the spin coupler (“spin dilution”), or by increasing the size of the ferromagnetic coupler, which in turn reduces the contact zone between magnetic orbitals. In both cases, |J| decreases. Thus, designing chemical structures for arylamine magnetic compounds requires finding a proper balance between spin bearers and spin couplers’ sizes. Moreover, considering specifically the structure 13463


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The Journal of Physical Chemistry B Scheme 2. Synthesis of Intermediate Compounds Containing 3,4′-Biphenyla

(a) Bis(4-butylphenyl)amine, Pd2dba3, BINAP, t-BuONa, toluene, 110 °C; (a′) NBS, DMF; (b) and (b′) BuLi, THF, −78 °C, isopropoxy-4,4,5,5tetramethyl-1,3,2-dioxaborolane, RT; (c) 3-bromoiodobenzene, Pd(PPh3)4, Ag2CO3, THF, 70 °C; (d) Pd(OAc)2, BINAP, amine S6, t-BuONa, toluene, 110 °C. a

and the corresponding exchange coupling constant J has been computed by DFT in the case of the trimer compound 1.

states for dimer d and trimer t, respectively. The exchange coupling constant J estimated by DFT calculations and determined by magnetization measurements showed a high value of 135 K. In particular, as we observed the coupling of three spins in the case of the linear trimer t; this suggested that it would be possible to achieve magnetic spin interaction along the corresponding polymer chain as well. In this paper, we report the synthesis and spectroscopic studies of the polymer PB1, composed of 4-butylphenylamine spin bearers and containing 3,4′-biphenyl spin couplers as for the previously reported oligomers d and t (Scheme 1). We also report the synthesis and spectroscopic studies of a new family of linear arylamines composed of 1,4-phenylenediamine (PD) (spin bearer) moieties connected by 3,4′-biphenyl couplers (Scheme 1). This family includes first the trimer model compound 1 (equivalent of trimer t for PB1) as well as two derived linear polyarylamine-based polymers, PB2 and PB3, containing 1,4-PD moieties built both in main chains and as pendant chains, respectively. All compounds have been oxidized both electrochemically and chemically to obtain radical cations. The oxidation process and the formation of radical cations were monitored by UV−vis spectroscopy. The spin interaction was investigated by pulsed-EPR spectroscopy. Magnetization was measured by the use of SQUID technique,

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RESULTS AND DISCUSSION Synthesis. The trimer model compound 1 as well as the polymers PB1−3 were obtained using palladium-catalyzed Buchwald−Hartwig and Suzuki coupling reactions. The polymer PB1 contained arylamine group, that is, 4-butylphenylamine separated via the 3,4′-biphenyl coupler. To decrease both the electrostatic repulsion and the oxidation potential, we have also prepared the polymers PB2 and PB3 containing the 1,4-PD unit built in the main and pendant chains, respectively (Scheme 1). The key step was the synthesis of arylamine with two 3,4′-biphenyl moieties (Scheme 2). The dibromo derivatives 2a and 2b were prepared starting from different substrates. The bromination of 4-tert-butylphenyl-diphenylamine with NBS led to dibromo derivative 2a, whereas in the case of 2b, tris(4-bromophenyl)amine was reacted with bis(4butylphenyl)amine in the presence of Pd2dba3/BINAP. The dibromo derivatives were converted to the corresponding pinacol borate derivatives 3a and 3b with 56 and 60% yields, respectively. The Suzuki coupling reaction of the compound 3 with 3-bromoiodobenzene in the presence of Pd(PPh3)4 and Ag2CO3 allows preparing the compounds 4a and 4b with 71 and 82% yields, respectively. Then, amination of the dibromo 13464


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The Journal of Physical Chemistry B Scheme 3. Synthesis of the Polymers PB and the Model Compound 1a

a (a′) 4-Butylbromobenzene, Pd(OAc)2, t-Bu3P, t-BuONa, toluene, 110 °C; (b′) bis-N,N′-(4-butylphenyl)-1,4-phenylenediamine, Pd(OAc)2, t-Bu3P, t-BuONa, toluene, 90 °C; (c′) Pd(OAc)2, t-Bu3P, t-BuONa, toluene, 90 °C.

solution contained particles of diameter 9.0 ± 0.2 nm (100%) determined from the size distribution by number and 11.8 ± 0.3 nm (99.6%) determined from the size distribution by volume. Electrochemical Properties. The electrochemical properties of all of the compounds containing 1,4-PD units were studied by cyclic voltammetry in dichloromethane with 0.1 M Bu4NBF4 as a supporting electrolyte. PB1 was poorly soluble in CH2Cl2 solution containing electrolyte; therefore, this polymer was studied as a thin film on a Pt electrode in 0.1 M Bu4NBF4 in acetonitrile. The cyclic voltammogram of the polymer PB1 showed one quasi-reversible wave at 0.62 V versus Ag/Ag+ (Supporting Information, Figure S1A). This value was slightly higher than that (0.55 V versus Ag/Ag+) measured for the first oxidation potential of the amine trimer t with the 3,4′-biphenyl coupler, reported previously.36 This peak can be attributed to the oxidation of amine sites linked at meta position to 3,4′biphenyl moieties. However, the second oxidation potential of PB1 was poorly defined and appeared ∼0.73 V versus Ag/Ag+. The very hydrophobic chains of this polymer probably do not favor the accumulation of charges introduced upon oxidation. More reversible systems than PB1 were observed for the compounds containing p-PD moieties. The cyclic voltammogram of the trimer model compound 1 showed two reversible waves at 0.19 and 0.67 V versus Ag/Ag+ (Supporting

derivatives 4 with 4-butylaniline or with N,N-bis(4-butylphenyl)-1,4-phenylenediamine afforded corresponding compounds 5a and 5b containing two active secondary amine groups (with the yields of 95 and 74%, respectively). The model compound 1 was obtained in the Buchwald− Hartwig reaction of the compound 5b and 4-butylbromobenzene (Scheme 3). The polymers were prepared by the polycondensation reaction. Thus, the condensation of 4a and 5a led to the polymer PB1, whereas the condensation of 4b and 5b afforded the polymer PB3. The coupling of 4b and N,N′bis(4-butylphenyl)-1,4-phenylenediamine gave the polymer PB2. In all of these cases, the catalytic system Pd(OAc)2/tBu3P was used. The low molecular weight fraction was removed from the polymers by extraction with hexanes, following the precipitation in methanol. In the case of PB1, Mn = 102 kDa and Mw = 133 kDa were determined by size exclusion chromatography in dichloromethane using polystyrene as a standard. However, this method failed in the determination of molecular weights for the polymers PB2 and PB3. Thus, we used dynamic light scattering (DLS) measurements performed in CH2Cl2. DLS showed that in the case of PB2, the solution contained particles of diameter 5.1 ± 0.3 nm (99.7%) determined from the size distribution by number and 5.5 ± 0.3 nm (93.3%) determined from the size distribution by volume. In the case of PB3, the 13465


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The Journal of Physical Chemistry B Information, Figure S1B). Both peaks are large with Eox − Ered = 160 and 150 mV, respectively, indicating that multielectron oxidation processes took place. The shapes of both peaks in the differential pulse voltammogram (Figure 1A) are not

The cyclic voltammograms of the polymers PB2 and PB3 (Figure 1B and Supporting Information Figure S1B-E) showed exactly the same values of the oxidation potentials. Both voltammograms are similar to that already recorded for trimer 1. The oxidation processes were reversible and appeared at 0.24 and 0.69 V versus Ag/Ag+. The first oxidation peak can be attributed to the formation of one radical cation per PD unit, similarly to the oxidation of 1. However, in these cases, the peak shape was symmetric, and we could not observe differences between the oxidation of PD units built to 3- or 4′- positions of biphenyl couplers. It may be caused by the polymeric nature of PB2 and PB3, which implies that the vast majority of PD units have a similar local environment in the polymer chains. This contrasts with the trimer 1, in which the difference between the central PD unit and the lateral PD units is more pronounced. As emphasized above, the second oxidation peak located at 0.69 V can be related to the oxidation of PD units into imine dications. However, the current ratio for the first and the second oxidation peaks was found to be 2:1.5 for PB2 and 2:1 for PB3. This indicates that not all PD units of the polymers were oxidized to dicationic form. UV−Vis−NIR Spectroscopy. The chemical oxidation of all of the compounds was followed by UV−vis−NIR spectroscopy. The compounds were oxidized with tris(4-bromophenyl)ammonium hexachloroantimonate (TBA·SbCl6). The spectra of neutral compounds were measured in CH2Cl2 solution. The spectrum of neutral PB1 showed one band at 322 nm (ε = 3.48 × 104 M−1 cm−1). This polymer was oxidized in CH2Cl2 solution due to some precipitation in the presence of acetonitrile. After the oxidation of PB1 to one radical cation per mer, its absorption spectrum changed significantly, and new bands located at 490 and ∼1840 nm appeared (Figure 2A). The NIR band can be attributed to an intervalence charge-transfer band. Similar spectral characteristics were also observed previously for model compounds, namely, for the amine dimer d and trimer t.36 The oxidation of PB1 to two radical cations per mer changed the absorption spectrum again; therefore, the NIR band was displaced to ∼1620 nm, and a new very intense band appeared at 740 nm. This new band can be related to the transition between orbitals centered on carbon atoms toward empty orbitals centered on nitrogen atoms. Similar evolution of the UV−vis spectra was observed previously for the model compounds.36 The absorption spectra of neutral compounds containing PD units were also measured in CH2Cl2 solution and showed one band at 327 nm (ε = 4.54 × 104 M−1 cm−1) for 1, 341 nm (ε = 6.17 × 104 M−1 cm−1) for PB2, and 344 nm (ε = 4.97 × 104 M−1 cm−1) for PB3. The oxidation of the neutral compounds was performed using TBA·SbCl6 solution in acetonitrile. The evolution of absorption spectra for all of these compounds after their oxidation was very similar (Figure 2B,C). The spectra of 1 and PB3 oxidized to one radical cation showed the appearance of new bands located at 417 and 923 nm or 419 and 933 nm, respectively (the spectra of oxidized PB2 can be found in Supporting Information, Figure S2). Upon further oxidation of PD units to radical cations, the spectra showed a significant increase of the intensities of both bands, which reached their maxima for 1 oxidized with 3 equiv of the oxidant and for PB3 oxidized with 2 equiv of the oxidant per mer. For higher oxidation levels than that mentioned above, the spectra changed their character; the intensity of the band at 930 nm decreased and new bands appeared at 513, 705, and ∼1500 nm in the spectrum of 1 and 510, 752, and ∼1500 nm in the

Figure 1. Differential pulse voltammogram of 1 (A) and PB2 (B) in CH2Cl2 solution (the concentration of the compounds was c = 10−3 M) containing an electrolyte, 0.1 M Bu4NBF4 (reference electrode Ag/ 0.1 M AgNO3 in acetonitrile; scan rate: 100 mV/s).

symmetric, and two shoulders located at ∼0.28 and 0.76 V can be observed. We can compare this voltammogram to that obtained previously for the trimer t containing 3,4′-biphenyl couplers and three 4-butylphenylamine sites.36 We can observe that, as expected, the insertion of PD units into the structure of the trimer 1 decreased significantly the oxidation potentials. The first oxidation peak registered in the voltammogram of t appeared at 0.55 V (versus Ag/Ag+) and was attributed to the oxidation of two lateral amine sites. Thus, the first peak at 0.19 V in the voltammogram of 1 can also be related to the oxidation of lateral PD units. This peak is however shifted by 0.36 V with respect to the oxidation potential measured for t. We can suppose as well that the oxidation potential of the central PD unit of 1 was also shifted with respect to the potential measured for t (0.70 V). It can be noticed that the difference between the oxidation potentials of PD units separated via 1,3-phenylene is 0.15 V.35 A similar difference (0.15 V) was observed between the oxidation peaks of the trimer t. Thus, in the case of 1, the difference between the oxidation potential of lateral and central PD units cannot be higher, and this oxidation process should be visible as a shoulder at ∼0.28 V. The second oxidation wave in the voltammogram of 1 was located at 0.67 V, with the shoulder at ∼0.76 V. This oxidation corresponds also to a multielectron process and can be related to the oxidation of PD units into imine dications. Similarly, as was postulated above, the oxidation corresponded to the formation of dicationic forms in both lateral and central PD units. 13466


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The Journal of Physical Chemistry B

Figure 3. 2D field-swept pulsed-EPR nutation spectra of PB1, 1, PB3, and PB2 and 1D pulsed-EPR nutation spectrum of a reference S = 1/2 sample. PB1, 1, PB3, and PB2 were oxidized with TBA·SbCl6 in CH2Cl2/CH3CN solution ([1] = 5 × 10−3 M and [Ox]/[1] = 3; [PBn] = 7.5 × 10−3 M and [Ox]/[PBn] = 2, with n = 1, 2, or 3). The reference sample for S = 1/2 is made of the polyarylamine PA213 at low doping level ([PA2] = 10−2 M and [Ox]/[PA2] = 0.5). All of the nutation experiments were performed at T = 7 K (see the Supporting Information for experimental details).

CH2Cl2/CH3CN (1:1) solution. The stoichiometry of the oxidation was adjusted so that 1 equiv of oxidant was added per equivalent of spin-bearing unit in the compound to be doped. Higher or lower oxidation stoichiometry led to lower spin states (data not shown). For the polymer PB1, a dominant S = 1/2 state is observed, and signals corresponding to higher S = 1 and 3/2 spin states are small. Several attempts were made in order to reach higher spin states for doped PB1 (data not shown); different solvents were used (e.g., nitromethane or pure CH2Cl2), and oxidation using SbCl5 was investigated. In all cases, only a dominant S = 1/2 state with a very small amount of higher spin states was observed. For the trimer 1, the 2D pulsed-EPR nutation spectrum exhibits a dominant signal at νnut = √3·ν0, corresponding unambiguously to the S = 3/2 spin state. This spectrum shows that the oxidation of this trimer is quantitative and demonstrates that the obtained triradical trication has a S = 3/2 ground spin state. In the case of the polymer PB3, the 2D pulsed-EPR nutation spectrum is very similar to the spectrum of the trimer 1, and the dominant signal has a nutation frequency between √3·ν0 and 2·ν0. This could be due to a mixing of S = 3/2 and 2 states in the sample or to a sample containing mainly S = 3/2 but investigated with a slightly increased quality factor of the EPR cavity that would accordingly increase the value of ν0. Overall,

Figure 2. (A) UV−vis−NIR spectra of PB1 oxidized with TBA·SbCl6 in CH2Cl2 solution (the concentration of the polymer was c = 6 × 10−4 M); Ox/PB1 molar ratio: (a) 1; (b) 2. UV−vis−NIR spectra of 1 (B) and PB3 (C) oxidized with TBA·SbCl6 in CH2Cl2/CH3CN solution (the concentration of 1 was c = 3.8 × 10−4 M, and the concentration of PB3 was c = 2.2 × 10−4 M); Ox/1 or PB3 molar ratio: (a) 0; (b) 1; (c) 2; (d) 3; (e) 4; (f) 6.

spectrum of PB3. These changes are related to the oxidation of PD units to the imine dicationic state. It should be emphasized that the evolution of the oxidation process using UV−vis−NIR spectroscopy is in perfect agreement with the conclusions arising from electrochemical studies. Pulsed-EPR Nutation Spectroscopy. The chemical oxidation of the compounds, trimer 1 and polymers PB1−3, by TBA·SbCl6 was studied by pulsed-EPR nutation spectroscopy. The nutation frequencies measured by this technique are directly related to the spin state values of the paramagnetic species in the sample by eq 1 given in the Supporting Information. Frequencies corresponding to different spin multiplicities are indicated by gray lines in the spectra reported in Figure 3. The 2D pulsed-EPR nutation spectra shown in Figure 3 were obtained at T = 7 K for the polymers PB1, PB2, and PB3 as well as for the trimer 1, all oxidized with TBA·SbCl6 in 13467


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function corresponding to a pure S = 3/2 spin state. It includes a T−θ term corresponding to mean field analysis of small antiferromagnetic intermolecular interactions. The value of θ = −0.72 (±0.1) K is consistent with the analysis of the χT = f(T) experiment (see below). From this analysis, the number of S = 3/2 species can be obtained, and it appears that (100 ± 3)% of the initial molecules of 1 have been doped up to triradical trications, thus creating the S = 3/2 spin state. The variations of magnetic susceptibility with temperature were recorded at low magnetic field (H = 0.01 T) and produced the χT = f(T) curve shown in Figure 4 (lower frame). This curve was modeled by eq 1 derived from the Van Vleck formula corresponding to a linear trimer of S = 1/2 spins.37,38 It is assumed that in the triradical trications obtained from the trimer 1, two S = 1/2 electronic spins are localized in both extremities (spins noted S2 and S3) and one in the central part of the molecule (spin noted S1). Because of the symmetry of the spin’s distribution, it could be then modeled by the Heisenberg Hamiltonian H = −(J·S1·S2 + J·S1·S3), leading to the following equation for fitting the χT = f(T) curve:

this spectrum still demonstrates that at least S = 3/2 has been reached in this polymer but, in the absence of a clear signal at a √6·ν0 nutation frequency, the presence of the S = 2 state is not established. Finally, in the case of the polymer PB2, the dominant signal has a nutation frequency of 2·ν0, and a small but clear signal can be observed as well at √6·ν0. The smaller signal observed at √6·ν0 is clearly due to both |2,−1⟩ ↔ |2,0⟩ and |2,0⟩ ↔ |2,1⟩ EPR transitions of an S = 2 state. The signal observed at 2·ν0 can therefore be attributed to both |2,−2⟩ ↔ |2,−1⟩ and |2,1⟩ ↔ |2,2⟩ EPR transitions of an S = 2 state. Although this last signal could also be attributed to the |3/2,−1/2⟩ ↔ |3/2,1/2⟩ EPR transition of an S = 3/2 state (see the Supporting Information), the presence of this dominant S = 3/2 would have resulted into an even stronger signal at √3·ν0 due to both |3/2,1/2⟩ ↔ |3/2,3/2⟩ and |3/2,−3/2⟩ ↔ |3/2,−1/2⟩ EPR transitions as for trimer 1. The absence of a strong signal at √3·ν0 enables us to conclusively rule out the possibility of a dominant S = 3/2. Therefore, the features observed by pulsedEPR nutation spectroscopy for PB2 can be attributed to a dominant S = 2 spin state, similar to the case of another S = 2 ground-state doped polymer that we reported previously.35 SQUID Magnetometry. A sample of the oxidized solution of the trimer 1 with [Ox]/[1] = 3, similar to that studied by pulsed-EPR nutation spectroscopy, was studied by SQUID magnetometry. These measurements are corrected for diamagnetism (see the Supporting Information). The experimental M = f(H) curve recorded at T = 2 K is shown in Figure 4 (upper frame). The experimental curve is very well fitted by a Brillouin

χT =

Ng 2β 2 10 + exp(−J /2kT ) + exp(−3J /2kT ) T . (T − θ) 4k 2 + exp(−J /2kT ) + exp(−3J /2kT )

(1)

From this analysis, the exchange coupling constant within the triradical trication derived from 1 was estimated to be J/k = 11.8 ± 5 K, and the Weiss temperature of the mean field analysis was θ = −0.72 ± 0.1 K. DFT Calculations. For the trimer 1, we computed bonding energies for the HS state S = 3/2 (HS = ↑↑↑) and for two socalled broken symmetry (BS) states (Ms = 1/2) derived from HS by slipping either the central spin (BS1 = ↑↓↑) or one of the two symmetrical external spins (for example, BS2 = ↓↑↑, which is of the same energy as BS3 = ↑↑↓). It can be then shown that J ≈ E(BS1) − E(HS), 2(E(BS2) − E(HS)), or 2(E(BS1) − E(BS2)) (see the Supporting Information). These three relations are numerically applied to the bonding energies computed for HS, BS1, and BS2 (see Table 1), resulting in the same value,J = 0.0028 eV = 23 cm−1 = 33 K. Table 1. HS and BS States Constructed for the Trimer 1 and DFT-Bonding Energies (eV) Computed with the B3LYP XC Potential spin states

eV (cm−1)

HS (S = 3/2) BS1 (Ms = 1/2) BS2 (Ms = 1/2)

−1220.5388 −1220.5360 −1220.5374

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DISCUSSION Strength of the Ferromagnetic Spin-Coupling Constants in Model Trimers. The ferromagnetic spin interaction is reflected by the positive value of the exchange coupling constant J, the magnitude of which depends on the chemical structure of organic compounds. The modest experimental value J/k ≈ 12 K obtained in this study for the trimer 1 is ∼2 times lower than that estimated using DFT calculations. Such a discrepancy is here quite acceptable considering the small magnitude of J, resulting from differences of otherwise large energies. The two magnetic orbitals computed for BS1 and BS2 are shown in Figure 5. It can be seen that they both occupy the same meta-substituted phenyl ring of the biphenyl spin coupler

Figure 4. Magnetization measurements of a sample obtained by chemical oxidation of 1. Molar ratio [Ox]/[1] = 3 and [1] = 4.5 × 10−3 M in CH2Cl2/CH3CN solution. Upper frame: M = f(H) curve recorded at T = 2 K. Lower frame: χT = f(T) curve recorded for 1 at H = 0.01 T. 13468


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Figure 5. Magnetic orbitals plotted for BS1 (left) and BS2 (right). Isodensity value: 0.03 au. For each magnetic orbital, a small insert focusing on the meta-phenyl of the 3,4′-biphenyl coupler (isodensity value: 0.02 au) is shown (larger images focusing on the meta-phenyl group are provided in Figure S3). Notice that these meta-phenyl parts have been rotated to ease their visualization. A detailed (but technical) comment on their mutual comparison has been reported in the Supporting Information.

Table 2. Highest Spin Multiplicities in Linear and Branched Doped Polyarylamines

attributed to partial oxidation of the polymer chain caused by an increased oxidation potential for 4-butylphenylamine moieties when surrounded by other doped 4-butylphenylamine moieties. It could also be due to the strongly hydrophobic nature of the chain, which probably impedes contact with the rather polar oxidizing agent. Bushby et al. reported that a doped branched polyarylamine based on the same kind of phenylamine moieties and 3,4′-biphenyl couplers exhibited only S = 1/239 (see Table 2). The fact that the oxidizing agent could not efficiently diffuse into such a branched polymer was there proposed. However, our unsuccessful attempts to reach HS states by doping the linear PB1 pleads rather for too high redox potentials of arylamine moieties in the polymer chain due to electrostatic repulsion and/or for very different conformations in the doped polymer chains compared with those of oligomers d and t that would not be suitable for ferromagnetic interactions. The trimer 1 and polymers PB2 and PB3 were therefore synthesized using a PD unit in order to lower both electrostatic repulsion and redox potentials in the polymer chains. The pulsed-EPR nutation spectrum of oxidized PB3 showed S = 3/2, and the spectrum of PB2 even revealed S = 2. In these polymers, the ferromagnetic spin interaction now spreads along linear chains and extends over three bearers and two couplers in the case of PB3 and over four bearers and three couplers in the case of PB2. The difference between both polymers is associated with the location of the PD unit, which can be viewed as belonging to the main chain frame for PB2 and as

(see insets zooming in on this meta ring). A detailed (but technical) comment on their mutual comparison as it relates to the magnitude of J has been reported in the Supporting Information. Let us recall that, although the DFT calculations have been performed without consideration of the influence of counteranions and explicit solvents molecules (as in previous reports34,36), these do not affect significantly the computed J values. From this point of view, the relative agreement between both J values (calculated and experimental ones) is quite satisfactory. It should be emphasized however that the experimental exchange coupling constant value J obtained for the trimer 1 is 1 order of magnitude lower than that obtained previously for the trimer t containing 3,4′-biphenyl coupler and three amine sites (Scheme 1). The chemical structures of both trimers differ only by the nature of their respective spin-bearing units; the arylamine sites in trimer t were replaced by 1,4-PD moieties in compound 1. Thus, though increasing the size (through conjugation) of the spin-bearing unit increases the chemical stability of the radical cation, this also causes spin density dilution and strongly decreases the magnitude of spin coupling. Spin Multiplicity in Doped Oligomers and Polymers. Despite the promising high ferromagnetic J value observed for doped trimer t and dimer d, the linear polymer PB1 built with 4-butylphenylamine moieties connected via 3,4′-biphenyl couplers did not exhibit any HS state; only S = 1/2 was observed by pulsed-EPR nutation experiments. This could be 13469


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The Journal of Physical Chemistry B “pendant” group for PB3. Probably the spin distribution is more favorable for PB2 (i.e., closer to the ferromagnetic coupler) than it is in the case of PB3, in which the spin density is shifted out of 3,4′-biphenyl. Two sets of comparable polyarylamines (see Table 2) have been reported to date. The first is doped polyarylamines reported by Bushby et al.39,40 based on phenylamine moieties connected by 4,4″-metaterphenyl couplers. The branched polymer exhibited a HS state (S = 5/2),39 but no linear polymer was reported. The second one is the doped polyarylamine reported by our group based on N-p-Ph-N-p-Ph-N moieties connected via 1,3-phenyl couplers, but this lead only to S = 1 states for the linear polymer34 and to a mix of spin states for the corresponding branched polymers (S ≤ 2).41 Therefore, it should be emphasized that both PB2 and PB3 doped polymers exhibit the highest spin states observed to date for linear polyarylamine compounds.

Applications in Electroluminescent Devices. Chem. Rev. 2009, 109, 897−1091. (2) Sekine, C.; Tsubata, Y.; Yamada, T.; Kitano, M.; Doi, S. Recent Progress of High Performance Polymer OLED and OPV Materials for Organic Printed Electronics. Sci. Technol. Adv. Mater. 2014, 15, 034203. (3) Sirringhaus, H. Device Physics of Solution-Processed Organic Field-Effect Transistors. Adv. Mater. 2005, 17, 2411−2425. (4) Wu, W.; Liu, Y.; Zhu, D. π-Conjugated Molecules with Fused Rings for Organic Field-Effect Transistors: Design, Synthesis and Applications. Chem. Soc. Rev. 2010, 39, 1489−1502. (5) Lee, W. Y.; Giri, G.; Diao, Y.; Tassone, C. J.; Matthews, J. R.; Sorensen, M. L.; Mannsfeld, S.; Chen, W. C.; Fong, H. H.; Tok, J. B. H.; et al. Effect of Non-Chlorinated Mixed Solvents on Charge Transport and Morphology of Solution-Processed Polymer FieldEffect Transistors. Adv. Funct. Mater. 2014, 24, 3524−3534. (6) Boudreault, P. L. T.; Najari, A.; Leclerc, M. Processable LowBandgap Polymers for Photovoltaic Applications. Chem. Mater. 2011, 23, 456−469. (7) Duan, C.; Huang, F.; Cao, Y. Recent Development of Push−Pull Conjugated Polymers for Bulk-Heterojunction Photovoltaics: Rational Design and Fine Tailoring of Molecular Structures. J. Mater. Chem. 2012, 22, 10416−10434. (8) Ye, L.; Zhang, S.; Huo, L.; Zhang, M.; Hou, J. Molecular Design toward Highly Efficient Photovoltaic Polymers Based on TwoDimensional Conjugated Benzodithiophene. Acc. Chem. Res. 2014, 47, 1595−1603. (9) Bujak, P.; Kulszewicz-Bajer, I.; Zagorska, M.; Maurel, V.; Wielgus, I.; Pron, A. Polymers for Electronics and Spintronics. Chem. Soc. Rev. 2013, 42, 8895−8999. (10) Dougherty, D. A. Spin Control in Organic Molecules. Acc. Chem. Res. 1991, 24, 88−94. (11) Bushby, R. J.; Gooding, D.; Vale, M. E. High-Spin Polymeric Arylamines. Philos. Trans. R. Soc., A 1999, 357, 2939−2957. (12) Rajca, A. Organic Diradicals and Polyradicals: From Spin Coupling to Magnetism? Chem. Rev. 1994, 94, 871−893. (13) Rajca, A.; Shiraishi, K.; Pink, M.; Rajca, S. Triplet (S = 1) Ground State Aminyl Diradical. J. Am. Chem. Soc. 2007, 129, 7232− 7233. (14) Rajca, A.; Olankitwanit, A.; Wang, Y.; Boratynski, P. J.; Pink, M.; Rajca, S. High-Spin S = 2 Ground State Aminyl Tetraradicals. J. Am. Chem. Soc. 2013, 135, 18205−18215. (15) Gallagher, N.; Olankitwanit, A.; Rajca, A. High-Spin Organic Molecules. J. Org. Chem. 2015, 80, 1291−1298. (16) Kaneko, T.; Makino, T.; Miyaji, H.; Teraguchi, M.; Aoki, T.; Miyasaka, M.; Nishide, H. Ladderlike Ferromagnetic Spin Coupling Network on a π-Conjugated Pendant Polyradical. J. Am. Chem. Soc. 2003, 125, 3554−3557. (17) Ishida, T.; Iwamura, H. Bis[3-tert-butyl-5-(N-oxy-tertbutylamino)phenyl] nitroxide in a Quartet Ground State: a Prototype for Persistent High-Spin Poly[(oxyimino)-1,3-phenylenes]. J. Am. Chem. Soc. 1991, 113, 4238−4241. (18) Kanno, F.; Inoue, K.; Koga, N.; Iwamura, H. Persistent 1,3,5Benzenetriyltris(N-tert-butyl nitroxide) and its Analogs with Quartet Ground States. Intramolecular Triangular Exchange Coupling among Three Nitroxide Radical Centers. J. Phys. Chem. 1993, 97, 13267− 13272. (19) Tamura, M.; Nakazawa, Y.; Shiomi, D.; Nozawa, K.; Hosokoshi, Y.; Ishikawa, M.; Takahashi, M.; Kinoshita, M. Bulk Ferromagnetism in the β-Phase Crystal of the p-Nitrophenyl Nitronyl Nitroxide Radical. Chem. Phys. Lett. 1991, 186, 401−404. (20) Sugawara, T.; Matsushita, M. M.; Izuoka, A.; Wada, N.; Takeda, N.; Ishikawa, M. An Organic Ferromagnet: α-Phase Crystal of 2-(2′,5′dihydroxyphenyl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazolyl-1oxy-3-oxide (α-HQNN). J. Chem. Soc., Chem. Commun. 1994, 1723− 1724. (21) Ito, A.; Sakamaki, D.; Ino, H.; Taniguchi, A.; Hirao, Y.; Tanaka, K.; Kanemoto, K.; Kato, T. Polycationic States of Oligoanilines Based

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CONCLUSIONS This study shows that linear compounds composed of a 1,4-PD moiety altered with a 3,4′-biphenyl unit can be effectively oxidized to radical cations. The value of the exchange coupling constant between doped 1,4-PD moieties connected with a 3,4′-biphenyl unit was determined for the model compound trimer 1 both experimentally using magnetization measurements and computationally using DFT, yielding values of ∼12 and 33 K, respectively. The incorporation of 1,4-PD moieties into linear polymers significantly limits the Coulombic repulsion between neighboring holes, stabilizes the chemical structure of radical cations, and allows for the formation of HS states. Thus, the oxidized polymers of this type can even form quartet or quintet spin states. Such HS states are observed in linear arylamine compounds for the first time.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcb.5b08390. Synthesis procedure and spectroscopic characterization of all compounds, electrochemistry of the trimer and the polymers, details of pulsed EPR nutation, SQUID experiments, and DFT calculations (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Phone: 33-438783598 (V.M.). *E-mail: [email protected]. Phone: 48-22-2345584 (I.K.-B.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS I.K.-B., I.W., and L.S. wish to acknowledge financial support from the Foundation for the Polish Science (TEAM/2011-8/ 6). A part of the ADF calculations was carried out in the Wroclaw Centre for Networking and Supercomputing, WCSS, Wroclaw, Poland, http://www.wcss.wroc.pl, under Grant No. 283.

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REFERENCES

(1) Grimsdale, A. C.; Chan, K. L.; Martin, R. E.; Jokisz, P. G.; Holmes, A. B. Synthesis of Light-Emitting Conjugated Polymers for 13470


Article


in Alternating Branched Polyarylamines. J. Appl. Phys. 2011, 109, 074911/1−074911/6. (42) Michinobu, T.; Inui, J.; Nishide, H. m-Phenylene-Linked Aromatic Poly(aminium cationic radical)s: Persistent High-Spin Organic Polyradicals. Org. Lett. 2003, 5, 2165−2168. (43) Kulszewicz-Bajer, I.; Gosk, J.; Pawlowski, M.; Gambarelli, S.; Djurado, D.; Twardowski, A. Ferromagnetic Spins Interaction in Poly(m-p-aniline). J. Phys. Chem. B 2007, 111, 9421−9423. (44) Ito, A.; Taniguchi, A.; Yamabe, T.; Tanaka, K. Triplet State of Würster’s Blue-Based Di(cation radical). Org. Lett. 1999, 1, 741−743. (45) Yoshizawa, K.; Tanaka, K.; Yamabe, T.; Yamauchi, J. Ferromagnetic Interaction in Poly(m-aniline): Electron Spin Resonance and Magnetic Susceptibility. J. Chem. Phys. 1992, 96, 5516− 5522. (46) Ito, A.; Ota, K.; Tanaka, K.; Yamabe, T.; Yoshizawa, K. n-Alkyl Group-Substituted Poly(m-aniline)s: Syntheses and Magnetic Properties. Macromolecules 1995, 28, 5618−5625.

on Wurster’s Blue. Polycationic States of Oligoanilines Based on Wurster’s Blue. Eur. J. Org. Chem. 2009, 2009, 4441−4450. (22) Hirao, Y.; Ino, H.; Ito, A.; Tanaka, K.; Kato, T. High-Spin Radical Cations of a Dendritic Oligoarylamine. J. Phys. Chem. A 2006, 110, 4866−4872. (23) Ito, A.; Yamagishi, Y.; Fukui, K.; Inoue, S.; Hirao, Y.; Furukawa, K.; Kato, T.; Tanaka, K. Trimacrocyclic Arylamine and Its Polycationic States. Chem. Commun. 2008, 6573−6575. (24) Sakamaki, D.; Ito, A.; Furukawa, K.; Kato, T.; Shiro, M.; Tanaka, K. A Polymacrocyclic Oligoarylamine with a Pseudobeltane Motif: Towards a Cylindrical Multispin System. Angew. Chem., Int. Ed. 2012, 51, 12776−12781. (25) Sakamaki, D.; Ito, A.; Furukawa, K.; Kato, T.; Tanaka, K. Meta− para-Linked Octaaza[18]cyclophanes and Their Polycationic States. J. Org. Chem. 2013, 78, 2947−2956. (26) Wasserman, E.; Murray, R. W.; Yager, W. A.; Trozzolo, A. M.; Smolinsky, G. Quintet Ground States of m-Dicarbene and m-Dinitrene Compounds. J. Am. Chem. Soc. 1967, 89, 5076−5078. (27) Sugawara, T.; Murata, S.; Kimura, K.; Sugawara, Y.; Iwasaki, H.; Iwamura, H. Design of Molecular Assembly of Diphenylcarbenes Having Ferromagnetic Intermolecular Interactions. J. Am. Chem. Soc. 1985, 107, 5293−5294. (28) Fujita, I.; Teki, Y.; Takui, T.; Kinoshita, T.; Itoh, K.; Miko, F.; Sawaki, Y.; Iwamura, H.; Izuoka, A.; Sugawara, T. Design, Preparation, and Electron Spin Resonance Detection of a Ground-State Undecet (S = 5) Hydrocarbon. J. Am. Chem. Soc. 1990, 112, 4074−4075. (29) Ovchinnikov, A. A. Multiplicity of the Ground State of Large Alternant Organic Molecules with Conjugated Bonds. Theor. Chem. Acc. 1978, 47, 297−304. (30) Li, S. H.; Ma, J.; Jiang, Y. S. Strength of Spin Coupling in HighSpin Organic Molecules. J. Phys. Chem. A 1997, 101, 5567−5573. (31) Li, S. H.; Ma, J.; Jiang, Y. S. Electron Correlation and Magnetism: A Simple Molecular Orbital Approach for Predicting Ground-State Spins of Conjugated Hydrocarbons. J. Phys. Chem. A 1997, 101, 5587−5592. (32) Bushby, R. J.; McGill, D. R.; Ng, K. M.; Taylor, N. Disjoint and Coextensive Diradical Diions. J. Chem. Soc., Perkin Trans. 2 1997, 1405−1414. (33) Bushby, R. J.; Kilner, C. A.; Taylor, N.; Vale, M. E. Disjoint and Coextensive Amminium Radical Cations: a General Problem in Making Amminium Radical Cation Based High-Spin Polymers. Tetrahedron 2007, 63, 11458−11466. (34) Maurel, V.; Jouni, M.; Baran, P.; Onofrio, N.; Gambarelli, S.; Mouesca, J. M.; Djurado, D.; Dubois, L.; Jacquot, G.J.-F.; Desfonds, G.; Kulszewicz-Bajer, I. Magnetic Properties of a Doped Linear Polyarylamine Bearing a High Concentration of Coupled Spins (S = 1). Phys. Chem. Chem. Phys. 2012, 14, 1399−1407. (35) Dobrzynska, E.; Jouni, M.; Gawrys, P.; Gambarelli, S.; Mouesca, J. M.; Djurado, D.; Dubois, L.; Wielgus, I.; Maurel, V.; KulszewiczBajer, I. Tuning of Ferromagnetic Spin Interactions in Polymeric Aromatic Amines via Modification of Their π-Conjugated System. J. Phys. Chem. B 2012, 116, 14968−14978. (36) Maurel, V.; Skorka, L.; Onofrio, N.; Szewczyk, E.; Djurado, D.; Dubois, L.; Mouesca, J. M.; Kulszewicz-Bajer, I. Ferromagnetic Spin Coupling through the 3,4′-Biphenyl Moiety in Arylamine Oligomers − Experimental and Computational Study. J. Phys. Chem. B 2014, 118, 7657−7667. (37) Bleaney, B.; Bowers, K. D. Anomalous Paramagnetism of Copper Acetate. Proc. R. Soc. London, Ser. A 1952, 214, 451−465. (38) Belorizky, E.; Fries, P. H. Exact-Solutions for Simple Spin Clusters with Isotropic Heisenberg Exchange Interactions. J. Chim. Phys. Phys.-Chim. Biol. 1993, 90, 1077−1100. (39) Bushby, R. J.; McGill, D. R.; Ng, K. M.; Taylor, N. p-Doped High Spin Polymers. J. Mater. Chem. 1997, 7, 2343−2354. (40) Bushby, R. J.; Taylor, N.; Williams, R. A. Ferromagnetic SpinCoupling 4,4″-through Metaterphenyl: Models for High-Spin Polymers. J. Mater. Chem. 2007, 17, 955−964. (41) Gosk, J.; Maurel, V.; Gambarelli, S.; Djurado, D.; Szymczak, R.; Twardowski, A.; Kulszewicz-Bajer, I. Ferromagnetic Spins Interaction 13471


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